Cytokine-induced killer cells are a heterogeneous ex vivo-expanded population of polyclonal T lymphocytes with a mixed T-NK phenotype. They express the CD3 molecule, the T-cell marker, and the CD56 molecule, the NK-cell marker, and they are non-major histocompatibility complex (MHC)-restricted cytotoxic cells. CIK were first described by Lanier et al. in 1986 in both human and murine tissues.
They are rare in the peripheral blood, with approximately 3% of lymphocytes found to be of this phenotype. CIK kill a wide variety of solid and haematologic tumors, furthermore they shown little cytotoxicity against normal tissues, including normal bone marrow. These features and their proliferative rate suggest that CIK may be efficient cytotoxic effector cells in adoptive immunotherapy strategies.

Ex vivo expansion and CIK phenoptype

CIK cells can be efficiently expanded in vitro from peripheral blood mononuclear cells (PBMC), bone marrow or umbilical cord blood precursors by the timed addition of IFN-γ, Ab anti-CD3 and IL-2. The IFN-γ, added on day 0, activates monocytes present in the culture, which produce IL-12 and provide contact-dependent CD58/LFA-3 stimulus to T cells. All this drives Th1 phenotype and cytotoxic ability of CIK. The Ab anti-CD3, added on day 1, is a mitogenic stimulus for cells, which can be expanded in the presence of IL-2. The IL-2, added every 2-3 days, is necessary to sustain the expansion of CIK cells, than it plays an important role in the development of anti-tumor activity.
After 3-4 weeks of culture, they show two subpopulations: CD3+CD56- and CD3+CD56+, the last is the major cytotoxic subset. The CD3+CD56- subset are early effector cells, whereas the CD3+CD56+ cells are terminally differentiated late effectors bearing CD45RO+; CD45RA+; CD27low; CD28low; CD62L-; CCR7+ phenotype.CIK cells express some natural killer (NK) receptors such as NKG2D, DNAX accessory molecule-1 (DNAM-1) and low levels of NKp30. Differently to NK cells, they do not express the CD16 (Fcγ receptor) surface molecule and thus are not capable of antibody-dependent cellular cytotoxicity (ADCC).

Citotoxic Activity and Molecular triggers of CIK

The CIK antitumor cytotoxicity has been demonstrated in vitro and in vivo against several cancers such as hepatoma, lung, ovarian , renal and gastric cancer, and leukaemia. As previously said, it is non-MHC restricted and non-ADCC dependent cytotoxicity, but involves the CIK-target cell conctacts which trigger perforin and granzyme degranulation pathway. Perforin and granzyme releases activate apoptosis cell-death pathways in tumor cells
The complete mechanism of tumor recognition by CIK cells has not been completely clarified but the main role is played by the interaction of their NKG2D receptor with its ligands on tumor cells. The main ligands recognized by NKG2D are MHC class I-related molecules A and B (MIC A/B) and the members of the unique long 16-binding proteins (ULBPs) that are highly expressed in a wide range of malignant or transformed cells. Also, NKG2D activity is associated with the upregulation of the adaptor DAP10, induced by the high dose of IL2 present in the culture.
Along with NKG2D, NKp30, DNAM-1 and the LFA-1/ICAM-1 complex are implicated too in tumor recognition. In particular it has been showed that CIK treated with anti-LFA-1 mAb had reduced cytotoxicity.

Preclinical study in vivo with murine models

In order to test non-MHC-restricted antitumor of human CIK cells in vivo the most adopted model is the severe combined immunodeficiency (SCID) mouse model. Ordinarily, in this murine model the growth of human tumor xenograft was inhibited by the administration of various doses of CIK cells. These preclinical studies in vivo gave good results: in murine models the effect of CIK cells is dose dependent and a significantly increased efficacy has been observed with higher doses of CIK cells and multiple treatments. Few hours after the infusion, CIK cells rapidly and persistently localize at tumor sites, expressing NKG2D ligands. Different data were produced by Chan et al., who described an efficient tumor killing mediated by CIK cells against primary ovarian cancer.

Clinical studies

On the basis of these experimental observations that demonstrate the potential clinical utility, phase I/II clinical trials with autologous CIK cells are currently under investigation in patients with advanced-stage lymphoma and metastatic solid tumors.An international registry for clinical trials with CIK cells was recently activated; the aim of this initiative is to set standardized, reliable criteria and guidelines to homogeneously analyze retrospective data and design the future CIK trials.
In this webside is possible to consult a real time updated table with a list of clinical trials with CIK cells.